1. Field of the Invention
The present invention generally relates to a page printer, a resolution converting method, and a variable-length reversible compression process, and more particularly, to a page printer such as a laser printer and an LED printer, a resolution converting method for achieving high image quality while reducing a memory capacity, and a variable-length reversible compression process for a bi-level image. The present invention is also directed to a printer part of a digital copy machine.
2. Description of the Related Art
Recently, resolution of a page printer such as a laser printer is increasing, and at present, a page printer of 600 dots/inch (DPI) is widely used. Therefore, a memory capacity of a frame buffer (page memory) in the page printer needs to be approximately 4 MB for an A4 paper with 600 DPI, and approximately 8 MB for an A3 paper with 600 DPI. Such increases of the memory capacity of the frame buffer have a large influence on product cost. However, the cost of the page printer is currently decreasing. Therefore, it is necessary to prevent the cost of the page printer from increasing due to improvement of the resolution.
For a reversible compression technique for bi-level image data, a variety of compression schemes are used. Widely used compression schemes include a modified Huffman (MH) coding scheme (one-dimensional compression scheme), a modified READ (MR) coding scheme (two-dimensional compression scheme), and a modified MR (MMR) coding scheme, which are used in a facsimile, etc., and a Lempel-Ziv (LZ) scheme which is usually used for a text file and a binary file in a computer. Furthermore, recently, a JBIG scheme using arithmetical coding which is internationally standardized is also used.
However, since these reversible compression techniques for the bi-level image data carry out a sequential coding process in sequence from a top of the image or the file, it is difficult to process only a desired area of the image or the file in real time.
In the computer, information of the image or the file is formed by a plurality of image information which are produced in selected areas on a CRT in sequence. Some of the image information may be produced in the same area as in a case of multiple windows. When the plurality of image information are transmitted to the page printer to be printed, they are transmitted in an order of the image information being sequentially produced. Therefore, to timely compress the plurality of the image information in the page printer, the image information in the desired area should be processed earlier. Accordingly, it is difficult to directly use the above-mentioned compression techniques for the page printer.
In other methods disclosed in publications "PCL 5 Comparison Guide for Printers", Hewlett Packard No. 5961-0999, first edition, May, 1993, and "SoftBand (registered trade mark) High-Performance Low-Memory Imaging", Proprietary Information, Phoenix Technologies Ltd., in the computer or the page printer, the plurality of image information are converted to a format which can be processed in sequence from a top of the file, whereby the above-mentioned compression techniques may be used. However, in this case, sometimes necessary memory capacity may overflow designed memory capacity, and, thus, the memory capacity may not be sufficiently reduced.
Further, at present, there is no fixed-length reversible compression technique for the bi-level image data in the world. Though the fixed-length compression technique may always reduce the memory capacity, this compression technique cannot recover an original resolution image (no reversible).
Accordingly, with the present compression techniques, any image data may not be compressed to a desired data amount.